During the reverse osmosis wastewater treatment process, approximately 25% of the effluent, or concentrated wastewater, is produced, which is discharged. Directly discharging the reverse osmosis concentrated wastewater would lead to water resource waste. However, how to handle the concentrated wastewater has always been a challenge. To treat reverse osmosis concentrated saltwater, a low-temperature evaporation-crystallization process is used, incorporating desalination technology. This process employs multi-effect evaporation to concentrate the high-salt concentrated wastewater, using the recovered evaporated freshwater as make-up water. The remaining solid salt residue from the evaporation and crystallization is then treated as secondary waste, achieving the recycling of reverse osmosis high-salt concentrated wastewater and zero discharge.
I. Current Status of Reverse Osmosis High Salinity Water Treatment
Currently, common desalination methods for high-salinity brackish water used both domestically and internationally include the following:
Enhance recycling rates
Direct or indirect discharge
3. Utilize comprehensively after various treatments.
4. Achieve zero emissions through evaporation concentration and pollutant removal.
Reverse Osmosis Brine Treatment Technology Process
Reverse osmosis brine treatment volume is quite large, characterized by its extremely high salt content. Based on zero discharge of reverse osmosis brine and the utilization of recyclable resources, the following process flow is adopted for treatment to achieve zero discharge.
Distillation - Crystallization Process
The treatment of reverse osmosis high-salt wastewater often employs a distillation-crystallization process. It is a desalination method that has evolved from desalination technologies based on seawater desalination. This technology involves heating the reverse osmosis saline wastewater to boiling point for evaporation, followed by condensing the steam into freshwater and further crystallizing the concentrate to produce salt. The technical basis for this method primarily includes multi-effect evaporation and steam condensation technologies.
2. Membrane Distillation and Crystallization Process
Combining membrane distillation separation technology with vacuum crystallization. Compared to other membrane separation processes, it has the following advantages:
High retention rate
2. Low energy consumption:
3. Simple equipment.
4. Capable of effectively treating high-concentration, high-salt wastewater that reverse osmosis equipment cannot handle. It also boasts energy-saving and environmental protection advantages, with the coupling of membrane distillation and crystallization as two separation technologies.
The membrane distillation process involves removing the solvent from the solution, concentrating the reverse osmosis brine to a supersaturated state for crystallization, and then obtaining crystals in the crystallizer. During this process, solvent evaporation and solute crystallization are completed separately within the membrane module and the crystallizer. This technology can utilize low-heat-value waste heat, saving energy consumption while operating at low temperatures. It imposes lower mechanical performance requirements on membranes and equipment, thereby reducing the total investment and maintenance costs.
3. Brine Low-Temperature Utilization – Evaporation-Crystallization Process
The utilization of low-temperature brine with saltwater, employing desalination technology, utilizes waste heat from cooling and the distillation concentration process to multi-effect evaporate reverse osmosis concentrated brine. The recycled condensate is used as the supplement water for the evaporation, and the residual salt sludge from the evaporation and crystallization process is further treated to achieve zero discharge and reuse of high-salinity wastewater.
Section 3: Water Resource Utilization Post-Reverse Osmosis Concentrate Treatment
1. The reverse osmosis brine recirculation method can effectively improve the recovery rate of reverse osmosis equipment; however, due to the high recirculation rate of reverse osmosis, it is prone to scaling, which can affect the lifespan of the RO membrane and increase costs.
2. The reverse osmosis brine will be reused as process water. The brine contains no suspended solids, is dosed with anti-scalant, and has pressure, making it suitable for backwashing of filters, dust removal, sludge and slag flushing, cooling water, or, after simple treatment, can be mixed with raw water for recovery.
The Significance of Research on Treating Reverse Osmosis Brine
The research on reverse osmosis brine treatment technology aims to address the issue of high-salt brine discharge in enterprises, promote the effective utilization of brine resources in various enterprises, and reduce or eliminate the discharge of reverse osmosis brine by using crystallization. This approach can decrease environmental pollution while enhancing the recovery rate of high-salt brine in reverse osmosis equipment, reducing the consumption of freshwater resources by enterprises, and effectively utilizing low-temperature waste heat, which contributes to the energy conservation and emission reduction efforts of enterprises.
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